
How exactly do you do it? Easy. Take a leaf or flower and place it carefully upside down on some fabric. (This won't be permanent, but as it's for a wall hanging I'm happy. You have to soak the fabric in an alum solution for permanence.) This is a busy Lizzie flower.
Tape it down with masking tape to keep it in place.
Turn the fabric over and whack it with a hammer! 
Take off the masking tape and mashed plant remains, and there you have it!

Here is my wreath as it's building up. The big splats are geranium leaves. Not completely successful, but good fillers!

Here is the completed wreath. I used some pieces of grass round the edges to give a bit of a feathery look to it.
I'm going to embellish it with embroidery and beads etc, so here's the start. Looking good, eh?